The present invention relates to circuits which are capable of eliminating crosstalk occurring between lines in a switching network for use in a telephone/data exchange system.
Heretofore, a switching network, used in a telephone/data exchange system, has eliminated crosstalk occurring between lines by either increasing the leakage resistance or reducing the stray capacitance, which are unavoidably present between the lines where crosstalk occurs. Alternatively, the prior networks have further reduced the imbalance of the ground impedance between two conductors constituting each line.
Meanwhile, along with the rapid progress attained in the application of semiconductor technology, crosspoint semiconductor switching elements have been developed, improved and applied to switching networks. However, limitations on manufacturing technology make it difficult to develop a semiconductor switching element having a sufficiently large leakage resistance or a sufficiently small stray capacitance. This resistance or capacitance is present between the terminals of semiconductor switching elements; therefore, it is often impossible to eliminate intelligible crosstalk which occurs between lines connected to the terminals of semiconductor switching elements. The crosstalk inevitably grows larger with increases in the size and scale of the switching network.
An unbalanced type of semiconductor switching network may have a greater variety than a balanced type network has, for economic reasons. Crosstalk between lines is particularly recognizable when a semiconductor switching elements are used in an unbalanced type of switching network. Accordingly, the more economically advantageous network is more detrimental to speech quality.
Efforts are often made to eliminate crosstalk by maximizing the leakage resistance and minimizing the stray capacitance between lines (i.e., between the terminals of semiconductor elements). However, in equipment depending upon such techniques, the maximization of the leakage resistance and the minimization of the stray capacitance seem to have reached their limits. For a disclosure of elimination of this kind of crosstalk, reference may be made to the switching network disclosed in U.S. Pat. No. 4,057,691, which eliminates crosstalk by grounding idle or unused lines, through impedance elements. However, such a system tends to have a smaller crosstalk elimination while the network is operating under heavy traffic loads, because the system design allows a very few lines to be left unused under such a grounded state, for economic reasons.